4" Demo Coil Spec.

 

Please note, the following information is merely a design summary for a particular system. It is presented "Out of interest" and it does not represent a set of plans which you should follow to construct a finished Tesla Coil....

LV SUPPLY

    

Supply voltage:

240

Vrms

Supply frequency:

50

Hz

Real power draw:

2550

W

VA draw:

2832

VA

VARs:

1232

VAr

Uncorrected power factor:

0.90

(lagging)

Optimal PFC capacitance:

68

uF

Supply current with PFC:

11.2

A rms

.

    

SUPPLY BALLAST

    

Ballast type:

Primary Inductive

  

Primary ballast inductance:

51

mH

Primary ballast resistance:

0.21

Ohms

Effective L referred to HV sec:

88

H

Effective R referred to HV sec:

361

Ohms

RMS voltage across ballast:

232

V rms

Average ballast voltage:

190

V avg

Peak ballast voltage:

554

V pk

RMS ballast current:

11.8

A rms

Peak ballast current:

17.0

A pk

Ballast I²R dissipation:

29.2

W

.

    

SUPPLY TRANSFORMERS

    

Type:

Oil filled plate xfmr

  

Primary rating:

230

V rms

Primary resistance:

0.24

Ohms

Primary inductance:

2.24

H

Secondary V rating:

9540

V rms

Secondary I rating:

200

mA rms

Secondary resistance:

460

Ohms

Secondary inductance:

3850

H

Transformation ratio:

41.48

  

Open circuit HV supply:

9955

V rms

Short circuit (ballasted) HV supply:

360

mA rms

RMS Primary voltage:

258

V rms

Peak primary voltage:

445

V pk

Average primary voltage:

210

V avg

RMS sec voltage when running:

10698

V rms

Peak sec voltage when running:

18465

V pk

Average sec volts when running:

8725

V avg

RMS primary current:

11.8

A rms

RMS secondary current:

284.5

mA rms

Primary I²R dissipation:

33.4

W

Secondary I²R dissipation:

37.2

W

Total I²R dissipation:

70.6

W

.

    

PRIMARY CAPACITOR

    

Tank capacitor value:

70.5

nF

Tank capacitor rating:

32

kV pk

Charging circuit Res. Freq.:

63.9

Hz

.

    

ROTARY SPARK GAP

    

Rotary type:

Salient pole synchronous

  

Motor speed:

1500

RPM

Motor rating:

1/8

HP

Disc diameter:

7

Inches

Electrode centres:

3

Inch radius

Flying electrodes:

1/8"

Zirc. Tungsten (x8)

Fixed electrodes:

1/8"

Zirc. Tungsten (x2)

Firing rate:

200

BPS

Electrode spacing:

Unequal

(stagerred 44/56%)

Electrode locations:

0, 40, 90, 130, 180,

  
 

220, 270, 310 degrees

  

1st firing point:

1.06 ms

after supply zero cross

2nd firing point:

6.62 ms

after supply zero cross

1st firing voltage:

18430

V

2nd firing voltage:

18430

V

1st bang energy:

12.0

J

2nd bang energy:

12.0

J

Total power throughput:

2400

W

.

    

TC PRIMARY WINDING

    

Winding type:

Inclined spiral

  

Material:

1/4" Soft Cu tube

  

Inclination:

15

Degrees

Inside diameter:

7

Inches

Outside diameter:

19

Inches

Turn spacing:

0.75

Inches

Number of turns:

8.5

  

Tap point:

7.5

  

Tapped inductance:

24.7

uH

Resonant frequency:

120.6

kHz

Primary tank surge impedance:

18.7

Ohms

Peak primary current:

986

A

RMS primary current:

45

A rms

Pri/Sec coupling coefficient:

0.18

  

.

    

TC SECONDARY WINDING

    

Winding type:

Closewound helix

  

Secondary diameter:

4.32

Inches

Secondary diameter:

21.3

Inches

Number of turns:

1500

  

Turns per inch:

70.4

  

Secondary inductance:

45.6

mH

Secondary resistance:

82

Ohms

Secondary self resonant freq:

272.1

kHz

Secondary self capacitance:

7.5

pF

Loaded sec resonant freq (small signal test):

126.0

kHz

Secondary surge impedance:

36.1

KOhms

.

    

DISCHARGE TERMINAL

    

Terminal type:

Hollow toroid

  

Toroid dimensions:

24" x 4"

  

Toroid capacitance:

27.5

pF

Total sec capacitance (without corona):

35

pF

.

    

DISCHARGE CHARACTER

    

Maximum spark length:

72

Inches

Average spark length:

54

Inches

 

 

ADDITIONAL COMMENTS:

 

The aim of this project was to produce a small demonstration coil, that would produce an arc of approximately 6ft in length from a secondary coil of under 2ft in length.

The coil produces a very characteristic sound due to the use of unequally spaced electrodes in the rotary gap. The tone is somewhere between that of a 100BPS and 200BPS gap, but appears harder and more aggressive.

I believe that the offset electrodes contribute significantly to the performance of this coil. In particular the unequally spaced electrodes have been found to delay the onset of saturation, allowing the power transformers to be worked harder before saturation starts to become noticeable.

200BPS operation was chosen for a number of reasons:

  1. It allowed more power to be delivered to the secondary before breakdown occurred due to excessive bang size. (too many Joules.)
  2. It provides two discharge instants for the tank capacitor per half cycle of the supply. This reduces the time period over which resonant rise can occur in the charging circuit. This minimises the chances of overvoltage due to prolonged resonance in the charging circuit.
  3. It was thought that 200BPS would keep the ionised arc channel sustained well, by delivering more energy every 5ms.
  4. 200BPS was found to represent the lowest firing rate that could produce fully coalesced streamers in an earlier test.
  5. 200BPS is the highest firing rate that can naturally produce two bangs of equal magnitude. Higher firing rates give rise to additional smaller bangs at lower voltages. This was thought to be inefficient use of the available power.

A fully synchronous design was chosen to prevent surging and allow accurate measurements to be taken.

It is also felt that this coil would have benefit from a slightly larger and much smoother toroid. At full power the toroids are covered with corona forming at edges and imperfections in the toroids. This represents a large area where power is being wasted. The relatively low 200BPS firing rate seems to demand a smooth toroid surface, although it is not nearly as critical as 100BPS operation.

Any comments are welcome,

-Richie,

 

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